The present invention is directed to multiple pass manifolds for tube bundles.
Large scale moisture separator-reheaters and other heat transfer equipment for steam power generation have found great utility in recent years. Such equipment typically employs horizontally arranged tube bundles having a large number of tubes which extend substantially the length of the device and return. Such equipment incorporates a header or manifold to direct steam or other heat transfer fluid to one end of the tubes of a tube bundle and collect the exiting vapor or condensate from the other end of the tubes.
Conventional manifolds have generally been designed to provide two pass and four pass systems. In a two pass system, the header requires a single baffle located to divide the header into two portions to separate the ends of each of the tubes. In this way, two passes substantially through the length of the heat transfer shell are possible.
In the standard four pass arrangement, an additional baffle is required to segregate a portion of the tubes normally associated with the first pass portion of the manifold in a two pass system. Thus, the first pass is made through, for example, the top half of the tube ends extending through the tube sheet. The bottom half of the tube bundle is retained as it was in the two pass system such that the fluid can exit from the tubes associated with the first pass system and re-enter the tubes blocked off from the first pass system in a counter flow. This arrangement results in the fluid flowing through a third and fourth pass to finally exit out of the ends of the tubes in the upper half of the tube bundle blocked off from the first pass.
A number of disadvantages exist with both the two pass and the conventional four pass systems. In the two pass system, it is relatively difficult to control the cooling rate in the tubes such that the condensate will not fill or block the heat transfer tubes. In the case of the conventional four pass system, the same difficulty of tube blockage arises partly because the third and fourth pass require the flow to move up rather than down in the tube bundle. In spite of the fact that the liquid condensate is eventually blown from the tube through continuous operation of the equipment, the formation, blockage and release of the condensate creates excessive thermal stress on the tubes and the associated equipment. Consequently, excessive deterioration has been experienced. In spite of the disadvantage of the four pass system, the four pass system allows for lower heat removal per pass and also for intermediate condensate removal. Both of these features allow better control of the tube condition with reduced slugging of subcooled condensate in saturated steam within the discharge legs of the tubes.
In part, the foregoing problems have resulted from the excessive size of current-day moisture separator-reheaters which have been developed for nuclear power plants. The systems include moisture separators combined with heat transfer units to dry and heat the low pressure steam prior to its entry into the low pressure turbine. With the dual functions of these devices and their excessive size, slugging and blowing of tubes is difficult to control.
To look in more detail to this thermal hydraulic instability resulting in severe oscillations in the tubes, it is believed to be a cause and effect syndrome. The cause may lie in the reheat bundle itself, where large steam volume diminishes into low liquid volume. This volumetric change, particularly at the lower portion of the bundle, is associated with changing hydraulic patterns as the two phase flow progressively transforms from annular flow at the tube inlet to slug flow at the flooded discharge. In addition, this oscillation-prone flow in the tubes is subject to erratic change of the phase transition point between the diminishing vapor core and the start of the flooding zone. The situation is aggravated by intermittent, inherent pressure perturbations associated with either bubble collapse or local flashing. On the other hand, oscillations may be triggered by external hydraulic perturbations not originating in the reheat bundle itself. Such external perturbations might include flashing in particularly full drain lines, operation of a control valve whose opening is not properly positioned to the set point deviation, or non-uniform discharge streams flowing into the exit due to higher condensation rates at the lower tubes. In any event, once such oscillation begins, the resulting syndrome is very difficult to control since the induced secondary oscillations may be of even larger amplitudes, and in the case of pressure fluctuations, once created, are often self-sustaining.
Further complications can arise from the fact that the zones flooded with increasing subcooled condensate are not of constant length. The zones are not of constant length because of lack of control over the discharge header pressure changes resulting from intermittent drainage of subcooled condensate and of saturated steam. Since the flooded links are inefficient heat transfer areas, they affect the shell side flow, causing local degradation in superheat levels. In turn, the temperature degraded zones increase the condensation rates within the tubes and thus cause secondary interference on the oscillating system.
Thus, the tube side two phase flow oscillations are inherently coupled with severe cyclic condensate subcooling which has been found to be proportionately greater in the lower portions of the tube bundles. The alternative discharge of saturated steam and slugs of subcooled water creates alternating temperature peaks along the discharge legs and around the tube-to-tube sheet joints which develop and decay in tandem with the discharge of saturated steam. The severity of such temperature peaks can result in thermal stresses which will cause detrimental fatigue failures to the tubes and to joints within the system.
The heating capability of the tube side flow has also heretofore lacked the versatility necessary to obtain a uniform heating of the shell side flow. As a result, losses in overall efficiency as well as excessive regional tube side subcooling can exist. Of particular concern are the regions of the shell side flow adjacent the sides of the shell. A greater amount of tube side heat is demanded in these areas.
Exemplified by development of the conventional four pass system mentioned above, the main course of refinement in moisture separator-reheater technology in recent years has been specifically directed to the stabilization of the two phase flow as a means to reduce the probability of tube failures and extend reheater service life. The following have operated as partial solutions or improvements in this area.
1. Flow restrictions in the reheater inlet header to equalize the flow through the tubes. PA1 2. Multi-tube-bank bundle, where the condensate is discharged at each bank exit while steam flows in series from bank to bank. PA1 3. High excess steam rate through all tubes. PA1 4. Employment of varying tube diameters on a horizontal configuration and withdrawal of non-condensing steam with some excess steam to normalize the hydraulic characteristics of all tubes and establish a discharge header pressure. PA1 5. Retrofitting existing two pass reheater bundles with internal headers which alter the condensing steam path from the two passes into four passes while draining the accumulated liquid phase at the discharge of the second pass and routing the combined two phase flow exiting from the fourth pass into a single discharge line. PA1 6. Providing internal secondary internal steam condensing coils which condense excess steam flow through the primary bundle.
Thus, a variety of designs have been attempted to resolve the aforementioned problems. One complete MSR design is illustrated in Yardin et al., U.S. Pat. No. 4,016,835, the disclosure of which is incorporated herein by reference.
In arriving at more complicated manifold requirements, greater sophistication is required in the header or manifold. Such added sophistication yields its own problems in sealing, thermal loading, flow distribution and the like. A natural division exists in the conventional tube bundles between the legs of the U formed by the tubes. However, if more than two passes are desired, more complicated sealing between closely spaced tubes is required. Thermal stress loading and flow distribution are also complicated by additional baffling.